CROSS REFERENCE TO RELATED PUBLICATIONS
This invention is related to U.S. Pat. No. 4,671,379 issued June 9, 1987 and assigned to the assignee of this invention.
1. Field of the Invention
This present invention is related to sources of generating seismic waves in the earth and more particularly to an apparatus for generating seismic waves substantially below the surface of the earth within a borehole.
2. Discussion of the Related Art
In the exploration of economic natural resources, it is desirable to determine as much as possible about the nature of the subsurface structure and lithology. Many techniques have been developed over the years in efforts to obtain such information. A commonly used technique is by using seismic exploration wherein a plurality of seismic detectors are disposed over the surface of the earth in a predetermined array. A seismic source such as a vibrator or pneumatic gun may generate an acoustic signal in the vicinity of the seismic detectors which receive the reflected acoustic signals from the subsurface strata. From this data, geologists and geophysicists are able to ascertain subsurface characteristics.
Once actual drilling of a well has begun in a selected area, it is typical to gather as much data as possible about the subsurface from the well. Drill rates and drill cuttings provide valuable information about the lithology. In addition, special instruments can be lowered down the borehole from time to time and the data analyzed. Another useful technique is to place an array of seismic detectors around the borehole at a predetermined radius and to generate a seismic signal deep in the borehole. This technique can provide valuable information concerning structure around the borehole as well as propagation velocities in the subsurface.
A major problem in the past for generating seismic signals deep in a borehole has been the kind of source to use. The use of explosives, once commonly used, now has limited application. The size of the explosive charge to provide the desired signal typically results in severe damage to the borehole itself. Another is the use of water or air guns, but the explosive pulses from these sources may result in damage to the borehole as well. Other techniques have been proposed such as dropping a heavy weight on the bottom of the borehole to generate the seismic signal, but this too may result in damage to the borehole.
None of the techniques mentioned above have proven entirely satisfactory and there remains a longstanding need within the industry for an improved apparatus to generate a powerful seismic signal in a borehole.